The NEXT experiment Status and prospects, J.J. Gomez-Cadenas on behalf of the NEXT collaboration October, 2015

Transcription

1 The NEXT experiment Status and prospects, J.J. Gomez-Cadenas on behalf of the NEXT collaboration October, 2015

2 NEXT: The collaboration IFIC,UPV,US,UdG, UAM, UZ Spain UC, UA Portugal UTA, UT-A&M, Iowa S, LBNL, FNAL USA

3 NEXT: A light TPC ENERGY PLANE (PMTs) TPB coated surfaces scintillation (S1) CATHODE e - e - e - e - e - e - ionization xenon gas electroluminescence (S2) ANODE EL mode is essential to get lineal gain, therefore avoiding avalanche fluctuations and fully exploiting the excellent Fano factor in gas TRACKING PLANE (SiPMs) It is a High Pressure Xenon (HPXe) TPC operating in EL mode. It is filled with 100 kg of Xenon enriched at 90% in Xe-136 (in stock) at a pressure of 15 bar. The event energy is integrated by a plane of radiopure PMTs located behind a transparent cathode (energy plane), which also provide t0. The event topology is reconstructed by a plane of radiopure silicon pixels (MPPCs) (tracking plane). 3

4 NEXT: Salient features Excellent resolution (~1% FWHM measured at 662 kev by NEXT prototypes, extrapolates to 0.5 % FWHM at Qbb Topological signature (TPS), eg. the ability to distinguish between signal ( double electrons ) and background ( single electrons ). Target = detector. Fiducial region away from surfaces. TPC: scalable. Economy of scale (S/N increases linearly with L) Xenon: the cheapest isotope to enrich in the market (NEXT owns 100 kg of enriched xenon). 4

5 The NEXT program DEMO (1 kg) NEW (10 kg) NEXT-100 (10O kg) ( ) Demonstration of detector concept ( ) Test underground, radiopure operation ( ) Neutrinoless double beta decay searches 5

6 DEMO (1 kg)

7 Hot Getter Gas System HHV modules NEXT-DEMO DAQ PMTs FEE SiPMs FEE 7

8 Energy Resolution Energy resolution measured with prototypes DEMO (IFIC) DBDM (LBNL) extrapolates to % FWHM at Qbb 8

9 Topological signature Signal events (bb0nu): TOP left MC event, two energetic blobs at the end of each electron (Bragg peak). Background events (Bi-214, Tl-208), single energetic electron, single blob, often with X-ray (xenon de-excitation) Bottom left, for signal event the energy of both blobs is high. Bottom right, for background events only one energetic blob. Topological signature: single track (no floating x-rays) with two energetic blobs: Signal efficiency ~50 %, background suppression 1% 9

10 Validation of TPS with DEMO Lower energy blob cand. enegy (kev) Higher energy blob cand. energy (kev) Proportion of events Higher energy blob cand. energy (kev) Figure 5. Energy distribution at the end-points of the tracks coming from 22 Na decay (left) and those coming from the 228 Th decay (right) for 2 cm radius blob candidates. 1. First proof of topological signature in high pressure xenon gas with electroluminescence amplification NEXT Collaboration (P. Ferrario et al.). Jul 21, pp. e-print: arxiv: [physics.ins-det] PDF Lower energy blob cand. energy (kev) Proportion of events TPS measured with DEMO data: background, Na-22 gammas, giving single electrons, MeV, signal ; Tl-208 double electrons (e- e+, double escape peak), MeV Analysis performed in Data and Monte Carlo simulation of DEMO with good agreement! First robust validation of Monte Carlo analysis for NEXT

11 NEW (10 kg)

12 NEW (NEXT-WHITE) at glance Time Projection Chamber: 10 kg active region, 50 cm drift length Pressure vessel: 316-Ti steel, 30 bar max pressure Tracking plane: 1,800 SiPMs, 1 cm pitch Energy plane: 12 PMTs, 30% coverage Inner shield: copper, 6 cm thick

13 NEW (NEXT-WHITE) at the LSC NEW on the seismic support table, inside the Lead Castle at the LSC

14 NEW field cage Field cage: 50 cm diameter, 50 cm drift length Poly boy, copper rings connected by low-background resistors

15 Energy plane 422mm# TOTAL#12#PMTs# 12 R PMTs (Hamamatsu) NEXT 100 will have 60 Excellent response (low noise very low dark current) in gas. Radiopure (less than 1 mbq/pmt in Tl-208 and Bi-214)

16 Tracking plane TP#NEW# 422mm# TOTAL#DICE# BOARDS#28# AND#1.792#SIPMs# 28 Kapton Dice Boards (KDBs) NEXT 100 will have ~100 Each KDB has 64 SiPMs from SENSL (thus, about 1,800 SiPms) SENSL SiPMs are the most radiopure currently in market,

17 Tracking plane: KDBs Made of low-background Kapton Long pigtail runs through 12 cm of copper shield. Connector BEHIND copper shield

18 NEW Schedule Energy plane (EP) installed in July Commissioning of EP in September. Tracking plane installation and commissioning October Mid November. Field Cage installation and commissioning: Mid November End of year. Commissioning run (full detector): First Quarter (Q1) Calibration run (energy resolution, topological signature, gas mixtures): Q2-Q Physics run (background model, bb2nu): Q1-Q

19 Energy Plane installation (July 2015)

20 NEXT-100 (10O kg)

21 NEXT 100 kg detector at LSC: main features Time Projection Chamber: 100 kg active region, 130 cm drift length Pressure vessel: stainless steel,15 bar max pressure Energy plane: 60 PMTs, 30% coverage Tracking plane: 7,000 SiPMs, 1 cm pitch Outer shield: lead, 20 cm thick Inner shield: copper, 12 cm thick 21

22 PERFORMANCE Energy Blob Candidate 2 (MeV) Energy Blob Candidate 1 (MeV) events / year energy (MeV) Energy Blob Candidate 2 (MeV) Energy Blob Candidate 2 (MeV) Energy Blob Candidate 1 (MeV) Energy Blob Candidate 1 (MeV) Figure 8. Energy spectra of signal (red, solid curve) and background ( 208 Tl: grey, dashed distribution; 214 Bi: grey, dotted distribution; total: grey, solid distribution) in the region of interest (ROI) around Q. The optimal ROI (the one that maximizes the ratio of the signal e ciency over the square root of the background rate) is indicated by the shaded, blue region. The signal strength represented here corresponds to a neutrino Majorana mass of 200 mev, while the backgrounds are scaled to their expected values in NEXT-100 ( counts/(kev kg y)), assuming an exposure of 91 kg yr. is defined as follows: Selection criterion Tl 214 Bi P (E 0 ) Fiducial, L = single P ( 208 track Tl) P (E 208 Tl) P 10( 214 Bi) P (E 214 Bi), (6.3) E 2 [2.4, 2.5] MeV where P (E H) is the probability of an event of energy E of being signal (H 0 ) Track with or background (H blobs Tl or H Bi). Figure 8 shows the distribution of signal and background around Energy ROI Q and the region of 3.89 interest 10 5 that maximizes0.457 the quantity "/ p b, selected usingtotal the likelihood ratio defined above Table 4 summarizes the acceptances for signal and background of the selection criteria Table 4. Acceptance of the selection criteria for 0 -decay described above. The natural radioactive backgrounds, 208 events described Tl and 214 in the text. The Bi, are suppressed by values for 208 Tl and 214 Bi correspond to one of the dominant sources of background in the detector. more than 6 orders of magnitude, and the contribution of 2 -decay to the background rate is 214 completely negligible. The cuts yield a signal e ciency of 28%. Note, however, Bi by the corresponding background rejection factors (defined as the inverse of the that approximately half of the events are lost already in the first selection cut: 88% of background acceptance resulting from the 0 -decay event selection described in the the events previous aresection). contained They within are also therepresented fiducial volume graphically of theindetector, Figure 9. 71% Thehave photosensors one single track, and 76% of them have reconstructed energy above 2.4 MeV (the 0 spectrum

23 Background rate Pressure vessel PMTs * PMT enclosures Enclosure windows EP support plate SiPM boards SiPMs * * * Table 6 shows the contributions grouped into six major subsystems. The background from 214 Bi is 4.3 times more abundant than the background from 208 Tl. The overall background rate estimated for NEXT-100 is < counts/(kev kg year). (7.1) This rate includes only radioactive backgrounds from detector materials and components. All other sources of background are expected to contribute at the level of 10 5 kev 1 kg 1 yr 1 or below: Field-cage barrel Shaping rings Electrode rings Anode plate FC resistor chain Inner shield * * Tl-208 Bi-214 background rate: 5 x 10-4 ckky WARNING: rate computed taking upper limits as actual values (a very conservative approach) Outer shield * * Background rate (10 5 counts kev 1 kg 1 yr 1 ) Figure 9. Contribution to the background rate of NEXT-100 of the di erent detector subsystems considered in our background model. An asterisk (*) next to a bar indicates that the contribution corresponds to a positive measurement of the activity of the material.

24 Radon Background rate (kev 1 kg 1 yr 1 ) Airborne Xe bulk Cathode Total internal Rn Activity (Bq/m 3 ) Figure 10. Background rate induced in NEXT-100 by airborne radon and radon contamination in the xenon gas (labelled as internal) in terms of the activity of 222 Rn. NEXT-100 will operate inside Radon-suppression tent (a la NEMO): expect ~200 mbq/m3 in air. Best guess for internal is tens of mbq/ m3. Contribution of Radon appears tolerable but needs to be understood by NEW operation

25 Sensitivity 10 8 EDF T 1/2 (10 25 years) 6 4 IBM- ISM mββ (mev) Expect 5 x y in 3 years run ( ) exposure (kg year) 100 mbb ~[90-180] mev depending on NME

26 Summary NEW at LSC: commissioning in 2015, operation in 2016 and DEMO analysis of Tl-208/Na-22 data with DEMO shows good topological separation and validates Monte Carlo calculations. NEXT sensitivity evaluated with last background model, results consistent with previous estimations. Expect a sensitivity to the period of y which translates in mbb ~[90-180] mev

27 The Future

28 R&D

29 How to improve the topological signature? Y (mm) SIGNAL Y (mm) BACKGROUND X (mm) X (mm)

30 Diffusion Charge (photoelectrons) mm mm σ_xy ~10mm/ m Drift Time (µs) Figure 15. Left: Average 3D charge distribution of a x-ray event from its barycenter. Right: S T, s x and s y of charge distribution gaussian fit versus Drift Length. 4.3 Transverse Spread (mm) S T The transverse response of the tracking plane to a point-like charge deposition is expected to have a width distribution due to the convolution of the transversal diffusion and the EL gap induced Transverse Spread S T. The ionization electrons will diffuse transversely as they drift up to the EL σ_z ~5 mm/ m region. Once there, due to the isotropic emission of light, each electron will be seen as the projection of a cone, and therefore, a K a deposit will be seen as multiple overlapping cones. Using the events selected as X-ray using the criterion mentioned above, a study of the extent of this projection was carried out. Figure 15-left shows the average projection of an event onto the x-y plane with the channel with maximum charge taken as the centre and the charge of the other channels plotted according to their distance from it. A two dimensional Gaussian can be fitted to the distribution to give an estimate of the transverse spread of the charge. Figure 15-right shows the sigmas of the two dimensional fit as well as their quadratic sum which is the parameter used to define the EL gap induced transverse spread (S T = p s 2 x + s 2 y ), plotted with drift time. There is no significant trend in the measured values with drift time suggesting that the EL gap distortion dominates the transverse spread of the charge cloud in the detector. The S T σ x σ y In pure xenon diffusion of drifting electrons is very large. After 1 m drift, the electron cloud has a transverse rms of the order of 10 mm and a longitudinal rms of the order of 5 mm Resolution is totally dominated by diffusion in NEXT.

31 TRUE 20 BACKGROUND 0.1 Y (mm) X (mm) Diffusion blurs the track reconstruction: The sharp wire with a blob of left panel gets blurred as diffusion increases.

32 How to improve resolution Resolution is dominated by longitudinal and transverse diffusion. It can be improved by adding small amounts of CO2. CH4 and CF4 may also be possible, but CH4 requires larger fractions of additive, which quenches more the light, and CF4 has side effects, such as dissociative attachment, TMA is excellent to reduce the diffusion but it appears to quench completely the xenon scintillation

33 CO2: 0.1 % (CH4: 1%) > DL < 2 mm CO2: 0.05 % (CH4: 0.5%) > DL < 2.5 mm

34 CO2: 0.1 % (CH4: 0.5 %) > DT < 1.5 mm CO2: 0.05 % (CH4: 0.25 %) > DT < 2 mm

35 Quenching of light CO2: 0.05 % quenches only 25 % of S1 CO2: 0.1 % quenches only 50 % of S1

36 The effect of difusion The effect of blurring the true track (left) by 2 mm diffusion (center) or 10 mm diffusion (right). In the example, the algorithm finds a fake blob in Bi-214 event for 10 mm diffusion but not for 2 mm diffusion

37 Mixtures: summary Overall, it appears possible to reduce diffusion to ~2 mm by adding 0.05 % of CO2. The side effects on reduction of S1 and drift velocity appear tolerable. R&D campaign foreseen for the next 6 months: Detailed study of the effect of gas mixtures (CO2, CH4) in NEXT performance. What is the compromise between diffusion and energy resolution?

38 Replace PMTs by SiPMs PMTs are rather radioactive, do not tolerate well high pressure and do not work in a magnetic field. But recent developments in SiPM technology make it possible to replace them! New generation SiPMs have dark current and dark noise at the level of khz/mm 2 at ambient temperature. Dark current decreases a factor 2 per 10 degrees celsius colder. Cooling SiPMs to -10 degrees appears feasible and results in ~10 khz/mm 2. OK for energy resolution

39 Symmetric TPC TRACK+CAL A (6 mm + 1 mm SiPMs) anode S1 e e e e S2 anode TRACK+CAL B (6 mm + 1 mm SiPMs) Replacing PMTs by SiPMs also allows to merge tracking and energy functions, by alternating SiPms of 1 mm 2 (for tracking) and 6 mm 2 (for energy). When an event is produced in the EL grid in front of plane B, SiPMs of 1 mm 2 (at a pitch of 7 mm) are readout for tracking, while SiPMs of 6 mm 2 (at a pitch of 7 mm) are readout for energy in plane A cathode S1 is readout by both planes and gives extra information of the localisation of the event (in addition to t0)

40 Upgrade NEXT Reduce diffusion to some 2 mm (e.g, 0.05 % of CO2) Symmetric TPC with mixed function planes. MC calculation shows that improvement of topological signature decreases background rate by a factor 4. PMTs are the dominant source in the background model. Replacing them by SiPMs could buy an extra factor of 2-3. Overall an addition rejection factor of 10, reaching 5x 10-5 ckky appears possible.

41 DEMO++ Field Cage and HVFT using NEW design to increase active volume. Tracking plane with 7 DICE boards for a better coverage of the active volume.

42 DEMO++ Only SiPM inside the detector. Energy measurement performed using 3x3mm2 SiPMs. Light concentrators (Wiston cones/lenses)

43 MAGIX Xe-136 decays produce Ba++ Ba++ will drift towards cathode (hopefully without recombining) Coat cathode with PSMA molecule, which will capture BA++ PSMA + BA++ will fluoresce when illuminated with 342 nm light (broad band, can design a system to detect blue light. Interrogation rate at ~100 khz. This idea is a new form of Ba-tagging in gas which does not involve extracting the Ba++ ion to vacuum. Potentially: background free experiment.

44 Alice experiment ent MAGIX Experimental tests planned by 2016! Ba++ by sparking Ba coated tips. Let Ba++ drift towards walls of a HP sphere. Illuminate with 330 nm led, and try to read 400 nm light.!!

45 Summary The NEXT project is moving forward. In particular, we have started the underground campaign with the commissioning of the NEW detector. New funding from Spanish ministry has been secured (1 M ). This adds to the existing AdG/ ERC grant. Funds from the USA (U. Texas at UTA Nygren) have also been made available at a significant level, reinforcing the very important US participation in the project. NEW will take data in 2016 and NEW will start operations in R&D on-going to understand effect of gas mixtures in TPS and the upgrade to a symmetric TPC readout by SiPMs only (upgrade DEMO prototype in 2016 and carry on studies). Test of performance in a magnetic field will follow (late 2016 or 2017). Also, start tests to understand the feasibility of the Molecule tagging concept. Even the most conservative upgrade (improving the TPS by reducing the diffusion, symmetric TPC) will result in a competitive detector for ton scale. Magnetic field or Molecule may result in a background free detector at the ton scale. The NEXT collaboration is seeking new partners to carry this ambitious program.